What do different execution contexts mean?
Probably you know more than me, but i think it is related to the words itself or the concepts behind it. 
Area, Screen and Region are pointing to where the oops are meant to be executed/invoked i think. "But dont know whats the diference between exec and invoke." now i do. thanks, (i dont use many scripted oops)
You know, Area, Screen and Region are the names blender gives internally to the the user interface diferent componets. see: https://docs.blender.org/manual/en/dev/interface/window_system/screens.html

https://docs.blender.org/manual/en/dev/interface/window_system/areas.html

https://docs.blender.org/manual/en/dev/interface/window_system/regions.html

hope useful.